Continuous casting



4 Sheets-Sheet 1 Filed Dec. 16, 1960 Jan. 5, 1965 J. l.. DEWEY 3,163,895

CONTINUOUS CASTING Filed Dec. 16. 1960 4 Sheets-Sheet 2 IN VEN TOR. fT/N L Dik/5) Jan. 5, 1965 3,163,895

J. L. DEWEY CONTINUOUS CASTING Filed Dec. 16, 1960 4 Sheets-Sheet 5 J. L. DEWEY 3,163,895

4 Sheets-Sheet 4 CONTINUOUS CASTING Jan. 5, 1965 Filed Dec. 1e. 1960 b om. 1 .N. N W W wb o! .ma wp. mm o o,/ c2 W m ,m m .mi W f om..." owwroz. .Bw ...o :525 mw ok L/o om oe /om MN /o o QQ Q /bwl m /o/ //////o 0.1 M IN2 umo mm o o m //G o /N/O/ w oww msi VIN HS o 8N United States Patent 3,163,895 CONTlNUUS CASTEJG lohn L. Dewey, Florence, Ala., assignor to Reynolds Metms Company, Richmond, Va., a corporation of Delaware Filed Dec. 16, 1969, Ser. No. 76,236 7 Claims. (Cl. 22-57.2)

This invention relates to the continuous casting of aluminum ingot of -a purity exceeding that of the molten feed metal.

For a long time there has existed the need for commercial aluminum ingot of high purity to satisfy the demand for high purity aluminum to be used in electric conductors and condensers, petroleum catalyst supports, corrosion-resistant chemical equipment, hardware, automotive trim, and, more recently, for use in nuclear re- -actor installations. The only process capable of supplying these needs heretofore has been the well known three-layer refining cell process and its modifications. This process, comprising the electrolytic refining of a copper-aluminum alloy in a fused salt bath at 73C-780 C., is presently in use throughout the world in spite of the fact that it costs nearly as much to refine a pound of metal as it does to produce the pound of metal initially from unmined bauxite. While many attempts have been made over the years to find a low-cost process to replace the expensive refining cell process, three-layer electrolytic refining has remained the one answer to todays commercial needs notwithstanding its inherently high cost burden. Substitute processes have proved useful only for laboratory purposes or in small-scale production of the high purity metal, and they too have imposed a high cost of production either because they require elaborate apparatus and a multiplicity of operating steps, or are not Well adapted to continuous production methods, or are notcapable of producing uniform high purity throughout the length of a continuously cast ingot, or for other reasons known to those who have been concerned with the problem. So, while some of these processes have found use in the purification of semi-conductor materials which s ell at hundreds of dollars `a pound, they are not continuous, seldom produce a uniform product, and are neither physically nor veconomically suitable for the purification of large quantities of low-cost metals, such as aluminum.

SUMMARY I have discovered a way to produce by a continuous casting method and apparatus, aluminum ingot having throughout its length a purity continuously maintained at a substantially uniform level above the purity of the molten feed metal. The production of this high purity, homogeneous ingot in commercial sizes according to my discovery can be accomplished at'only a fraction of the cost of electrolytic refining. I have also found that these ingots can be produced in any desired degree of purity by changing the production rate or the feed stock, so that the cost of production is in direct relation to the desired purity.

According to my invention molten metal is fed into the inlet of a pass-through mold in which a portion of the metal is frozen and the frozen portion is continuously withdrawn from the outlet of the mold. At the same time, the molten portion of the feed metal is caused to flow rapidly over the interface between the molten and frozen metal, as by stirring, and concurrently there is withdrawn from the mold as casting progresses molten metal including metal which has flowed over the interface and contains a higher percentage of impurities than is present in the feed metal. The relationship between (a) the rate of feeding the molten metal, (b) the rate of withdrawice ing the solid portion from the mold and (c) the rate of withdrawing molten metal from the mold, is adjusted so that each rate is maintained substantially constant relative to the others. This serves to maintain a substantially uniform composition of the molten metal just above the interface with a resultant uniformity in purity of the ingot produced. However, the rates can be increased or decreased as desired with purity of product changing inversely with the -rate of production. As an example, I have produced 8-inch diameter ingots, a size and shape 4commonly used for extrusion billets, from feed stocks of different purities at various production rates as shown in Table 1:

Table 1 Composition, Percent Produe- Aluminum Run No. tion Rate,

Feed Product Production equivalent to Run No. 7, if performed by the three-layer electrolytic refining process would require a specially constructed fused salt electrolytic cell, 62,000 amperes D.C. electricity with attendant rectification and distribution equipment, in addition to a direct chill casting unit to produce the ingot.

DESCRIPTION FIG. 4 is adiagram showing percentage removal of' metallic impurities in a typical run.

FIG. 5 is a-diagrarn showing total impurity `distribution lengthwise ofthe ingot product.

FIG. 6 is a diagram showing total impurity distribution transversely of the ingot product.

The continuous direct chill ingot casting apparatus constructed according to my invention and used in practicing my improved method comprises in its general arrangement the following principal parts: a pass-through moldv ing chamber 19 having an inlet and an outlet spaced longitudinally thereof, means which may include a heated pipe 11 for feeding molten metal into the upper end, or inlet, of the molding chamber, means which may include a Water spray 12 for freezing a portion of the feed metal, means which may include an ingot lowering device 13 for continuously withdrawing the frozen portion from the lower end, or outlet, of the molding chamber,-means which may include a stirring device 14 for causing the molten portion of the feed metal to ilow rapidly over the interface between the molten and frozen metal, and means which may include a heated overflow pipe 15 for continuously withdrawing from the molding chamber moltenmetal including metal which has flowed over the interface and contains a higher vpercentage of impurities than is present in the feed metal. Stirring as by means of the stirring de- 3 i vice 14 is performed in a manner that causes the molten metal adjacent the interface to rotate around the axis of the ingot while being drawn downwardly against the peripheral portions ,of the interface, thence inwardly towardY close several specific examples utilizing a preferred form of my continuous casting apparatus `with its improved means for maintaining a uniform rate of purification with resultant uniformity in purity of ingot produced.

EXAMPLE 1 Fifty-pound pigs 16 of. aluminum feed metal approxi-' matelyS x 6 x 28" long were tied onto an endless chain 17 by means of %diameter high purity aluminum rod 18. The chain was suspended over the melt crucible `19 by a sprocket 20 which in turn was connected through a gear system 21 to a variable speed motor (not shown) in such manner that the rate of rotation of Vthe sprocket, and hence the rate ofmovement ofthe chain, could be accurately controlled. As the sprocket revolved, the pigs 16 attached to the chain were lowered at a constant rate into the pool of molten aluminum 22 in the crucible, causing the level of the molten aluminum to rise above the inlet 23 of overflow pipe `11 so that liquid .aluminum flowed through the overflow pipe to the mold of the casting unit. As the pigs 16 contacted molten metal in the Crucible, they melted progressively from the lower end and were in due time completely released from the chain by melting of the 2K3" aluminum rod. When the pigs had been partially lowered into the melt additional pigs were hung from the chain in the same manner and spaced so as to maintain the total volume of aluminum at each point along the chain reasonably constant. Thus the rate of displacement of the molten aluminuminto the overflow pipe 11 was substantially constant.

The melt crucible 19 was a standard clay graphite tilting furnace crucible. It sat within a gas-fired furnace consisting of a silicon carbide tile combustion chamber 25, an outer steel shell 26 and a layer of thermal insulation 27 'disposed therebetween. The lrebrick bottom 28v of the furnace was covered with a layer of silicon carbide cement and the top cover 29 of the furnace was cast from Purotab alumina casting mix (approximately 85% tabular alumina, high-purity calcium aluminate).

A control thermocouple 30 was placed between the silicon carbide tile and the insulation in the position shown.

The signal from this thermocouple was read outon a standard .potentiometer-type recorder which also supplied a signal to a standard type of proportioning controller (not shown) which actuated a conventional servo motor arranged to operate the control valve of a gas burner 31 in accordance with the deviation of the furnace temperan ture from its set point.

The overtlow pipe 11 was constructed from la clay graphite tube spaced Within a tube 32 of silicon nitride bonded silicon carbide covered exteriorly with suitable thermal insulation. Hot combustion gases from the cast furnace 33 exhausted through the annularr space V34Y between the ytwo tubes, maintaining the temperature ofthe v clay graphite tube 11 above the melting point of the feed metal. The clay graphite tube and the clay graphite crucible 19 were shaped to form a ball and socket joint 35 which was coveredwith a thick layer of alumina cast? ing mix to prevent the joint from leaking. The opposite end of the tube was suspended over the upperportion of the cast unit so that aluminum feed owing from the melt furnace'through the overow pipe owed into the open top 36 of the mold 10.

The cast unit mold 10 was constructed inthe form of a vertically disposed graphite tube of 8" inside diameter mounted within a tubular clay graphite member 37 situated within the gas-heated cast furnace 33 of a construction similar to that of the melt furnace 24. A stirrer assembly 14 was suspended within `mold 10. The interior lip of the bell shaped upper end of mold 10 was filled with alumina casting mix 38 to provide protection from air burning above the molten aluminum level. The clay graphite overflow pipe 15 was attached to the graphite mold tube by a ball and socket joint 39 sealed in alumina casting mix. Molten aluminum overflowed through the pipe 15 into a Crucible 40 exteriorly of the furnace for removal from the plant as an overflow product. The clay graphite member 37 had a lower end 41 of reduced size 41 of member 37 was spaced slightly from mold 10 and' this space was filled with a commercial thermal insulation material comprising ceramic fibers with an inorganic binder known as Fiber-Frax cement. The furnace 33 was lired by a gas burner 44. A Chromel-Alumel control thermocouple 74 was maintained in a hole in 4the clay graphite member 37 in the position shown in FIGS. l

`and 2 to give a sensitive control of the temperature of the casting furnace. The signal from this thermocouple was read out on a potentiometer-type adjustable zero adjustable range recorder adjusted for a full-scale range of 2 mys. The recorder supplied a signal to a proportioning type controller which actuated a servo motor that positioned a gas control valve in accordance with the deviation of the control point temperature from its set point. The temperature variation during normal operation was 1% C. at the thermocouple junction.

The stirrer assembly 14 consisted of a C.S. grade graphite impeller section 45, a clay graphite shaft 46, drive means 47 for turning the shaft and irnpeller at predetermined constant speeds and means 48 for adjusting the vertical position of the stirrer assembly. The graphiteV graphite sleeve 51 was fitted around the extremities of the blades and held in position with graphite plugs flush with the outside surface of the sleeve. The upper end ot' shaft 49 was provided with a screw thread connection to the clay graphite shaft 46 secured to a steel collar 52..

The collar in turn was welded to a steel shaft 53, containing an elongated keyway 54. The shaft 53 extended through bearings 55 and was suspended by a cable 56 ex tending through a pulley 57 directly along the center line of the mold 10. Shaft 53 was keyed to the drive pulley 58 in a manner to permit the shaft to move vertically with respect to the pulley. The pulley was connected through a Vbelt to a variable speed motor 59 having infinitely variable speed settings between 55 and S50-rpm. The productk ingot 60 was lowered from the casting unit through the bottom end of the mold 10 by means of the hydraulic lowering device 13. The ingot shrank on cooling from the 8 diameter of the mold at the interface to about 71%6 diameter at its cool end. The freezing interface was usually about 16 up from the bottom of the mold. The water spray ring 12, positioned 10 below the bottom of the mold, consisted of a 11A pipe bent to form a 12" I.D. circle with sixty-four V16 diameter holes equally spaced along the intersection of a horizontal plane through the circle such that the water jets` were inclined downward about 30 from the horizontal plane. Water rate to the spray ring was metered.

The hydraulic lowering device 13 consisted of two sets of three-point clamps, each set built into horizontally disposed steel platforms 61, 62, supporting hydraulic cylinders 63, 64, for opening and 'closing the;l clamps and supported by vertically disposed hydraulic cylinders, 65, 66, respectively, for which the rate of downward movement was controlled and kept constant by withdrawing uid from the supporting cylinders at a constant rate with a metering pump. Pushbutton-type hydraulic valves 6'7, 68, placed so as to be operated according to the respective positions of the platforms when near the extremities of the platform motion controlled the tiuid ow directions from an auxiliary hydraulic pump and accumulator system (not shown) so that at least one of the sets of clamps was engaging the ingot and lowering at all times. When the upper platform neared theV end of its downward travel it contacted a pushbutton which started a sequence of operations, viz:

(7) The top platform began a downward motion at the y same rate as the lower platform. (8) The clamp set on the lower platform opened. (9) The lower platform-rose to its original position.

The ingot was therefore lowered continuously at a con-k stant controlled rate, which rate could be changed by changing the stroke length of the metering pump.V This type of lowering mechanism is described and claimed in the copendingvr application of Robert F. Treadway and Billy K. Davis, Serial No. 120,546, filed June'29, 1961.

The cutoff band-saw 69 consisted of a pair of wheels 72 mounted on axles 73 disposed parallel to each other and inclined upward to the horizontal, one `axle lying free in its bearings and the other laxle engaging a motor driving the band-saw blade at forty feet per second. The whole was suspended on a platform mounted on a' fixed track `and wheels so that the saw blade, which was held in place on the wheels by a pair of band-saw guides, could be pushed through the ingot to complete a cut as at C. The band-saw blade was a 1/2wide skip-tooth blade having four` teeth per inch. A maximum length of 17 could be cut from the ingot on one saw cut. Cuts were made wi-thout stopping the ingot lowering.

To start casting, -a 7%" diameter ingot was pulled into mold 10 by means of an eye bolt in the l.top of the ingot and was clamped into the lowering device 13, so that the top of the ingot was about even with the overflow pipe 15. The cast unit gas burner 44 was ignited and the controller set to maintaina temperature of 871.0 C. at the junction of thermocouple 74. Four hundred pounds of 99.90% aluminum was placed in the melt furnace 24, the gas burner 31 ignited :and the controller set to hold the melt furnace temperature at 775 C. The setting of the variable 4drive motor on the pig lowering unit was adjusted to give a lowering rate of the feed pig 16 of 11.2 per hour. The stroke length of the metering pump of the lowering device had been previously adjusted to give a lowering rate of the ingot 6i? of 1.0 inch per hour. After about eight hours, when the melt furnace crucible 19 wasl full of molten aluminum and the ingot 60 had melted down a distance of about l0" below the overflow pipe 15, the cooling water was turned on at a rate of about 11.8 cu. ft. per hour, the stirrer 14 was lowered into the molten aluminum with the bottom of its irnpeller 45 about 3" above the liquid-solid interface 79 and started rotating at 20() r.p.m. The bulkliquid temperature in the cast unit at this time was about 665 lC;

6 Lowering of the feed pigs 16 was then started. As so as molten aluminum began overflowing through the `overflow pipe 15 into the overflow Crucible 40, the lowering device 1 3 lwas started. Throughout the aboveV period the level of the liquid-solid interface 70 with respect to the mold 11@ was measured every fifteen minutes by probingV to ride up with the interface during an upset so that although there were occasions when the stirrer yactually rubbed on the interface it did not break or become embedded in the frozen aluminum. Normally the interface was quite stable at about the level of the furnace floor.

described conditions, the temperature of .the aluminum.

overowing from the melt furnace was 728 C., and the bulk liquid temperature in the cast unit was 672 C.

Inlet water to the spr-ay ring 12 was 15 C. and the runoif from the ingot was about 30 C.

When it was desired to shut the plant down, lowering of the feed pigs 16 was stopped, the lowering device 13 was shut off, the stirrer 14 was removed from the cast unit, cooling water was stopped and finally the gas burners 31 and 44 were shut off. Molten aluminum in the mold 1@ was allowed to freeze in place to form a portionof the starting ingot for a subsequent Similarly, molten aluminum in the melt furnace 24 was allowed to freeze in'place.

Eighty-three inches 'of high-quality ingot were made during the panticular pilot run here described.V Analyses showed that the impurity levels in the product were substantialiy less than in the feed, for instance, silicon 87% less, iron 94% less, copper 84% less, magnesium 67% less, zinc less, land gallium 55% less. See/FIG. 4.

Average analyses of the Product, Feed, and Overflow were:

Impurity Element; Feed, Product, Overflow, p.p.n1 ppm. p.p.m.

The distribution of the total impurities in the product ingot lengthwise of the eighty-three inch section is shown in FIG. 5. In this view each point shown by the graph represents the average of spectrographic analyses at between ten and seventeen points located in the same transverse plane. FIG. 6 shows distribution of total impurities transversely of the ingot. In this view each point shown EXAMPLE 2 In the manner described in Example 1, 19.7 -lbs'. per hour of 96% Al feed was passed through the cast unit and splitV into 7.0 lbs. per hour of approximately 7% inch diameter ingot and 12.7 lbs. per hour of overflow product. The ingot lowering rate was 1.4 inches per hour, the stirrer rate was 204 rpm., the distance between the liquid-solid interface and the bottom of the stirrer averaged 6.8 inches, and the cast unit control temperature was 878 C. rThe following average analyses were obtained Aover 55 inches of a continuous section of ingot:

Impurity Elements Feed, Product, Overflow,

p.p.m. p.p.m. p.p.m.

silicon 16, Go 7, 40o 21, 800 Iron 16, 200 5, 500 19, 900 Copper- 700 2, 400 6, 900 Zinc 230 200 360 EXAMPLE 3 Impurity Elements Feed,v Product, Overow,

p.p.m. p.p.m. p.p.m.

EXAMPLE 4 Additional gas burners were added to the melt furnace to provide a melting rate of 250 lbs. per hour. Feed of 99.9% Al was then passed through the cast unit at a rate of 250 lbs. per hour, where it was split into about 45 lbs. per hour of approximately 7% inch diameter ingot and about 205 lbs. per hour of 'overllow product. The ingot lowering rate varied between 8A and 10 inches per hour, the distance from the liquid-solid interface to the bottom of the stirrer was about 8.5 inches, the cast unit control temperature was 778 C., temperature ofthe molten feed to the cast unit was about 670 C., the solidliquid interface was 24 inches'below the overflow pipe 15, and the stirrer rate was 200 r.p.m. Average analyses over 30 inches of ingot were as follows;

Feed, Product, Overow p.p.m. p.p.m. YProduct,

Y p.p.m

EXAMPLE 5 8 stirrer was 10 inches. FDhe following analyses were obtained:

to illustrate a fairly wide range in the several operating parameters with respect to impurities in the feed metal, production rate and purity of the product ingot. It will be appreciated that there is a relationship between purity of feed and purity of product so that with higher purity of the feed metal there will be higher purity in the product ingot. Again, with a given purity'of feed metal, purity of the product will be influenced to some degree by production rate.k

Example 1 illustratesfoperations in which we utilize the highest commercial grade of aluminum` pig for the feed metal, using a low production rate and achieving a correlatively high product purity. Example 2 is illustrative of the application of my invention tocasting of purified ingot from feed metal below presently acceptable commercial grade. Example 3 is illustrative of the application of my invention in a -case where average grade pot metal is utilized. Example 4 illustrates conditions in which the feed purity isthe same as in Example 1 but production rate has been multiplied many times. Example 5 is similar to Example 1.

The terms and expressions which I have employed are used in a descriptive and not a limiting sense, and I have no intention of lexcluding such equivalents of the invention described as fall within the scope of the claims.

I claim:

l. Method of continuous casting of aluminum ingot having throughout its length a purity continuously maintained at a substantially uniform level above the purity level of the molten feed metal which comprises feeding molten metal into the inlet of a pass-through molding chamber, maintaining the molten condition of the metal in that part of the molding chamber which is nearest the inlet and maintaining a frozen condition of the metal iny that part of the molding chamber which is nearest the outlet, continuously withdrawing the frozen portion from the outlet of the `molding chamber while causing the molten portion to flow rapidly over the interface between the molten and frozen metal and withdrawing from the molding chamber at the molten metal side of the interface as "casting progresses a substantial portion of the molten metal including metal which has flowed over the interface and contains a higher percentage of 'impurities than is present in the Ifeed metal, the relationship between (a) rate of feeding molten metal, (b) rate of withdrawing the frozen portion from the molding chamber and (c) rate of withdrawing molten metal from the molding chamber, being maintained substantially constant each relative to the others.

2. Method of continuous casting of aluminum ingot having throughout its length a purity continuously maintained at a substantially uniform level above the purity level of the molten feed metalwhich comprises feeding molten metal into the inlet of a pass-through molding chamber, maintaining the molten condition of the metal in that part of the molding chamber which is nearest the inlet and maintaining a frozen condition of the metal in that part of the molding chamber which is nearest the outlet, and continuously withdrawing the frozen portion from the outlet of the molding chamber while stirring the molten metal adjacent the interface between the molten and frozen metal and withdrawing from the molding chamber at the molten metal side of the interface as casting progresses `a substantial portion of the molten metal including metal which has been stirred adjacent the aieaeos portions of the inter-lace, thence inwardly toward said axis and upwardly away from central portions of the interface.

4. Method of continuous casting of aluminum ingot having throughout its length a purity continuously maintained at a substantially uniform level above the purity level or the molten feed metal which comprises feeding molten metal into the inlet of a pass-through molding chamber, maintaining the molten condition of the metal in that part `of the molding charnber which is nearest the inlet and maintaining a frozen condition of the metal in that part of the molding chamber which is nearest the outlet, continuously withdrawing the frozen portion from the outlet of the molding chamber while causing the molten portion of the feed metal to flow rapidly over the interface between the molten and frozen metal and withdrawing a substantial portion of the molten metal from the molding chamber at the molten metal side of the` interface as casting progresses, the relationship between (a) rate of feeding molten metal, (b) rate of withdrawing the frozen portion from the molding chamber and (c) rate of withdrawing molten metal from the molding chamber, being maintained substantially constant each relative to the others whereby a substantially uniform composition of the molten metal just above the interface is maintained with a resultant uniformity in purity of the ingot produced.

5. Apparatus for continuous casting of high purity metal ingot having throughout-its length a purity continuously maintained at a substantially uniform level above the purity level of the molten feed metal which comprises a pass-through molding chamber having an inlet and an outlet spaced longitudinally thereof, means for t iti feeding molten metal into the inlet of said chamber, means for causing a portion of the feed metal to freeze at al point between said inlet and outlet, means for continuously withorawing the frozen portion from the outlet of the molding chamber, stirring means for causing the molten portion of the feed metal to ilow rapidly over the interface between the molten and frozen metal, and means for withdrawing from the molding chamber at the molten metal side of the interface as casting progresses a substantial portieri of the molten met including metal which has iiowed over the interface and contains a higher percentage of impurities than is present in the feed metal.

6. Apparatus for continuous casting of high purity metal ingot having throughout its length a purity continuously maintained at a substantially uniform level above the purity level of the molten feed metal which comprises a pass-through molding chamber having an inlet and an outlet spaced longitudinally thereof, means for feeding molten metal into 'the inlet of said chamber, means for causing a portion of the feed metal to freeze at a point y.between said inlet and outlet, means for continuously withdrawing the frozen portion from' the outlet of the molding chamber, means for stirring the molten feed metal adjacent the interface between the molten and frozen metal, and means for withdrawing from the moldchamber at the molten metal side of the interface as casting progresses a substantial portion of the molten metal including metal which has been stirred adjacent the interface and contains a higher percentage of impurities than is present in the feed metal.

7. Apparatus according to claim 6 in which the stirring means is constructed and arranged to rotate the feed metal adjacent the interface around the anis of the ingot while drawing the feed metal downwardly against the peripheral portions of the interface, thence inwardly toward said axis and upwardly away from central portions of the interface.

References Cited in the file of this patent UNITED STATES PATENTS 1,160,169 Hering Nov. 16, 1915 2,013,653 Hoke Sept. 10, 1935 2,708,297 Zeigler May 17, 1955 2,753,254 Rick July 3, 1956 

1. METHOD OF CONTINUOUS CASTING OF ALUMINUM INGOT HAVING THROUGHOUT ITS LENGTH A PURITY CONTINUOUSLY MAINTAINED AT A SUBSTANTIALLY UNIFLRM LEVEL ABLVE THE PURITY LEVEL OF THE MOLTED FEED METAL WHICH COMPRISES FEEDING MOLTEN METAL INTO THE INLET OF A PASS-THROUGH MOLDING CHAMBER, MAINTAINING THE MOLTEN CONDITION OF THE METAL IN THAT PART OF THE MOLDING CHAMBER WHICH IS NEAREST THE INLET AND MAINTAINING A FROZEN CONDITION OF THE METAL IN THAT PART OF THE MOLDING CHAMBER WHICH IS NEAREST THE OUTLET, CONTINUOUSLY WITHDRAWING THE FROZEN PORTION FROM THE OUTLET OF THE MOLDING CHAMBER WHILE CAUSING THE MOLTEN PORTION TO FLOW RAPIDLY OVER THE INTERFACE BETWEEN THE MOLTEN AND FROZEN METAL AND WITHDRAWING FROM THE MOLDING CHAMBER AT THE MOLTEN METAL SIDE OF THE INTERFACE AS CASTING PROGRESSES A SUBSTANTIAL PORTION OF THE MOLTEN METAL INCLUDING METAL WHICH HAS FLOWED OVER THE INTERFACE AND CONTAINS A HIGHER PERCENTAGE OF IMPURITIES THAN IS PRESENT IN THE FEED METAL, THE RELATIONSHIP BETWEEN (A) RATE OF FEEDING MOLTED METAL, (B) RATE OF WITHDRAWING THE FROZEN PORTION FROM THE MOLDING CHAMBER AND (C) RATE OF WITHDRAWING MOLTEN METAL FROM THE MOLDING CHAMBER, BEING MAINTAINED SUBSTANTIALLY CONSTANT EACH RELATIVE TO THE OTHERS. 